Multimode and single-mode polymer optical waveguides, which are rigid and formed on a printed board, or flexible and formed on a polymer base film, have been widely used. The principle of the polymer optical waveguide is that, depending on a combination of a core and a clad, which is a combination of a plurality of polymers having different refractive indexes, the core is set as an optical waveguide.
On the other hand, a silicon optical waveguide, which has an optical waveguide formed on a silicon chip, has been also widely used (refer to PTL 1).
In both of the polymer optical waveguide and the silicon optical waveguide, a plurality of waveguides are formed in an array manner in a form arranged in parallel along one direction in order to obtain a plurality of waveguide channels.
Although attempts have been made to propagate light between the polymer optical waveguide and the silicon optical waveguide, a high-precision positioning is required to realize a coupling of efficiently propagating light at a microlevel.
In the case of the multimode optical waveguide, in coupling between optical waveguides or between an optical waveguide and a multimode optical fiber, since the size of core cross-sections are large and both are similar to each other in the size of core cross-sections or the number of apertures, if accuracy of butt positioning of the cross-sections is ensured in the cross-sections to be in contact with each other, coupling can be realized with allowable level of loss.
Actually, this has been realized by so-called butt-coupling.
However, in coupling between a single-mode optical waveguide and a silicon optical waveguide, since core cross-sections become considerably small and both have significant differences in the size of core cross-sections or the number of apertures, butt-coupling itself becomes difficult.
Accordingly, a method expected as an alternative is an adiabatic-coupling that captures and communicates light (hereinafter, referred to as evanescent light) seeped out from an optical axis direction along the array, over a predetermined distance in the optical axis direction.
For this reason, attention has been recently paid to a composite optical waveguide obtained through an adiabatic-coupling of a polymer optical waveguide and a silicon optical waveguide (refer to PTLs 1 and 2).